I originally translated the first line as though the tui were referring to "bellas puellas", and so got a rather unconventional translation, transforming Sextus into a leno. >_> My mind has been corrupted by the one who says "esne forda vacca maris?"

A more conventional translation of the first line: You say that beautiful girls burn {with the love of you / with your love}.

I would say that "frater tuus" is probably a more natural way to say "your brother". However, "frater tui" means the same thing, so I guess you could say it.

In these "eius" is always masculine singular (referring to Marcus) - despite for example the plural subject "poma". This is because there is only one person doing the possession. The possessor is Marcus, who is male, so "eius" is always masculine (although in the singular, different-gendered pronouns all look the same anyway!) compare:

filius eorum bonus erat: their son was good
anima earum sacra erat: their (women!) soul was sacred.
pomum eorum laetissimum erat: their apple was happy.